It is well known to perform assays where aliquots of a sample are tested to see if the sample contains a particular substance by reaction with one or more reagents to form a complex such as an antigen/antibody complex which may then be detected.
A great number of different techniques exist for performing assays and the instances in which assay techniques are used are becoming more widespread. An overview of currently used techniques is given in the article "Update on immunoassay automation" Proc. UK NEQAS Meeting 1994:1:163-170; Wheeler, Michael J. For example, various types of immunoassays are now commonly used to test blood and other samples for a great number of different compounds. Yet, even a large hospital would not have a large number of different assay machines and so the apparatus must be capable of performing a range of different tests on different samples. The versatility of any assay technique must necessarily increase as the number of tests that can be requested on samples increases. The growing desire that assays can be performed directly in places such as Doctors Surgery's, rather than sending the samples away for analysis by a laboratory in a hospital further increases the demand for versatile machinery.
In conjunction with the growth of the assay systems, it has become necessary that any particular assay is performed ever more precisely. The importance of the accuracy of the test will be apparent, as, for example, a patient's treatment may be determined based on the result of the assay and so an inaccurate result may lead to inappropriate treatment of the patient.
The growth in assay techniques being used has led to a large number of systems on the market. Presently, most systems are semi-automated or automated systems, where after loading of the reagents and samples no further input is required from a human operator, unless a breakdown occurs.
As will be apparent from the aforementioned article by Wheeler, the majority of the devices presently on the market comprise a loading tray for loading multiple samples, for example between 20 and 100 samples, which are not necessarily of the same nature or having the same assay performed on them. There is also a reagent input tray which holds a number of reagent cartridges for the various different tests to be performed. In the machine the samples are transferred, normally by pipetting into an assay cell where the sample is combined with the necessary reagent or reagents. The assay cell is then transferred to a part of a machine where it can be held for sufficient time for the reagent and the sample to combine. Thereafter the sample cell is transferred to the detector which detects the presence of a known indicator to determine whether or not the sample contained a particular component and/or how much of that component was present in the assay. Normally a robotic arm is used for transferring the assay cell around the machine, for example from the loading area to the wash station, to the waiting after and onward to the detector. Whilst assay apparatus of this type can offer semi-automated functioning, problems do occur due to the mechanical movement of the samples. Furthermore, it is necessary to have pipettes with replaceable pipette tips or other means to ensure that one sample does not contaminate another sample when being. As each sample cell must be incubated with the appropriate reagents, a relatively large number of sample cells may have to be incubated at any one time and thus the size of the machine remains relatively large due to the space required for the waiting area.
Recently, it has been suggested to use flow injection technology in assay equipment. A review of this technology is given in an article by Puchades, R. et al "A Comprehensive Overview on the Application of Flow Injection Techniques in Immunoanalysis, Critical Reviews in Analytical Chemistry" 23,(4):301-321(1992). The fundamental difference between this technology and the above described systems is that the samples and the reagents are combined in a fluid stream, which extends to the detector. This dispenses with the need of sample cells and generally reduces the mechanical components of the system.
WO94/03104 discloses a flow injection technique with developments shown in WO94/20855 and WO95/22766. In this known flow injection technique, samples and reagents are again loaded into the machine. These are combined in the fluid stream. The fluid stream then passes along a conduit which has a transparent portion. In the transparent portion a screen is produced of small, preferably magnetic, beads. As the fluid stream carries these samples and reagents into the area of the screen, complexes of the reagent-sample are held by the screen. Normally, a second set of transparent beads are covered with a bound reagent complex which allows a competition assay to occur in this region. Quantitative analysis of the presence of a product in the sample is conducted by electromagnetic detection means, e.g. fluorometer, at this point which is why the beads and the section of the conduit must be transparent in this area. This technique is particularly suitable for measuring the rates and progress of the assay reaction.
Clearly, in this known flow injection method the detection means and the transparent area of the conduit must be in close physical proximity and it is suggested that the transparent part of the conduit is formed as part of the lens system of the detector means. This leads to a complex arrangement of the machinery which limits the types of assays which can be performed by this technique.
The present invention seeks to provide an improved assay system which is simple, reliable and versatile.